The present invention relates to a radiographic method and apparatus for irradiating an object with a radiation beam and obtaining a two-dimensional transmitted image with the radiation beam transmitted through the object. Although the present invention is particularly effective and will be described with reference to X-ray radiography hereinafter, the present invention is not limited to X-ray radiography but is widely applicable to medical diagnosis or industrial inspection of structures using .alpha.-, .beta.- or .gamma.-rays.
In X-ray radiography, when X-rays are transmitted through an object and reach a screen, they are considerably attenuated with respect to incident X-rays. X-rays incident on an object are considerably scattered at respective portions within the object and form scattered X-rays. For this reason, the transmitted X-ray image on the screen includes a scattered X-ray image component which largely lowers the contrast of the X-ray image. Various measures have been taken to eliminate scattered X-rays including, for example, the grid method, the Gradel method, and the slit scanning method. In the grid method, a grid is interposed between the object and the screen. Lead foil pieces are arranged in a stripe or matrix form in the grid. Scattered X-rays from the objects are attenuated by the grid. The attenuation factor of X-rays scattered from the object is larger than the attenuation factor of transmitted primary X-rays, so it is possible to improve the contrast of a transmitted X-ray image. However, if thin lead foil pieces are used, the eliminating effect on scattered X-rays is reduced as X-ray energy is increased. Furtheremore, in such a case, the grid also produces some scattered X-rays. Because of these reasons, the improvement in the contrast of the transmitted X-ray image is limited.
According to the Gradel method, the screen is located at a position a maximum possible distance from the object. In this case, since the sources of scattered X-rays are respective portions of the object, the intensity of the scattered X-rays is attenuated in proportion to the second power of the inverse of the distance between the screen and the scattered X-ray sources, i.e., the object. However, transmitted primary X-rays on the screen are also attenuated in proportion to the second power of the inverse of the distance between the X-ray source and the screen. In this case, the output of the X-ray source, i.e., the X-ray tube must be increased. This means an increased X-ray dose on the object. In addition, when the distance between the object and the screen is excessively increased, blurred resolution will occur in a transmitted primary X-ray image depending upon the size of the X-ray focal point of the X-ray source.
In the slit scanning method, a slit is interposed between the X-ray source and the object and another slit is interposed between the object and the screen. X-rays from the X-ray source are limited by the slit between the X-ray source and the object to form a fan-shaped beam which scans the object. The slit between the object and the screen permits the transmission of the fan-shaped beam. These slits are synchronously scanned in one direction with respect to the object. Thus, only the transmitted primary X-rays which pass through the object reach the screen. In the slit scanning method, the contrast of the transmitted primary X-ray image is improved in comparison with that obtained by the grid or Gradel methods described above. However, in the slit scanning method, the time required for scanning the slits is lengthy, resulting in long radiographic imaging times. Therefore, if the object is in motion during this time, primary transmitted X-rays from a single position in the object are recorded at different positions on the screen and form a blurred transmitted X-ray image. Consequently, in the slit scanning method, the slits must be scanned at high speed. An apparatus adopting this method thus becomes complex in structure and bulky. The radiation time of X-rays is increased, and the electrical load on an X-ray tube (the X-ray source) is increased.
In order to resolve the problems with these methods, Japanese Patent Application No. 12071/1984 (original Dutch Application filed on Nov. 26, 1974) proposed an X-ray radiographic apparatus. In this X-ray radiographic apparatus, a plate is interposed between an X-ray source and an object. The plate has an X-ray transmittance which changes along a predetermined pattern so as to spatially modulate the intensity of primary X-rays reaching the object. Transmitted X-rays are supplied to an image intensifier. An output image from the image intensifier is converted into electrical signals which are subjected to analog signal processing by a video circuit.
According to the principle of this X-ray radiographic apparatus, the intensity of the primary X-rays incident on the subject is modulated so as to allow discrimination between components of the transmitted X-ray image attributable to the primary X-rays and the scattered X-rays. More specifically, when primary X-rays are spatially modulated by the plate, actual video signals are obtained as a sum of certain portions of the modulated signal and the non-modulated signal. Thus, only the modulated component in the video signal is utilized to reproduce a transmitted primary X-ray image which is not influenced by scattered X-rays.
In the X-ray radiography described above, the video circuit performs the analog image processing. A video signal modulated by a plate is used to generate a reference signal, and analog image processing is performed using this reference signal. However, precise image processing is generally impossible, resulting in unavoidable ripple components at wave portions corresponding to changes in modulation factor of the plate. Due to the presence of these ripple components, the displayed image includes a noise component which hinders medical diagnosis or industrial inspection of X-ray radiography.